Bulbs, Corms, Rhizomes and Tubers

What Are Bulbs?

Essentially, bulbs are “storage tanks” that help plants survive dormant periods when it’s too cold or too hot for them to flower. They also nourish the plant during the flowering and growing season There are several major types of bulbs:

  • True bulbs (daffodils, tulips, hyacinths, etc.) are complete plants within a tiny package. They produce stems from the base of the bulb and survive from year to year. As the plant grows, bulblets form at the base of the mother plant.
  • Corms (crocus, freesia, and gladiolus) are usually short squat stems filled with food storage tissue. Some corms produce cormels which, like bulblets, are baby plants and can be separated from the parent to grow new plants.
  • Tubers (dahlias and some begonias) are underground roots with fleshy, food-storing parts that resemble tubers.
  • Rhizomes (irises) are bulb-like power packs that grow along the soil surface. Growth buds form on a rhizome for next year’s leaves and flowers. The original rhizome will not reflower and in time will need to be dug out.

Spring vs. Summer Bulbs

Spring-blooming bulbs are winter hardy. They are planted in the fall, grow and bloom in spring, then lie dormant for a year. Spring bulbs do not have to be dug up unless they need division. Most will continue to grow and bloom each spring, some for many years, some for only a year or two.

Some examples of spring bulbs are hyacinth, tulip and daffodil.

Summer-blooming bulbs are not winter hardy. They are planted in spring and they grow and bloom in summer. They can either be treated like annuals (enjoyed for a single season) or you can dig them up and store them in a frost-free area over the winter until they can be replanted the following spring. Some examples of summer bulbs are dahlias, cannas, gladiolas, and tuberous begonias.

Planting

All bulbs like sun, but keep in mind that many spring bulbs bloom before the trees leaf out, so you may be able to plant them in areas that become shady later in the season.

Prepare the soil before planting. The optimum pH range for bulbs is 6 to 7. A soil test of the planting area is necessary to determine if lime needs to be applied to adjust the soil pH.

Good soil drainage is essential. If you have a soil with high clay content, add compost, peat moss, or some other type of organic material. The organic material should be worked into the top 12-18 inches of soil.

Spring bulbs can be planted in late fall, up until the ground is frozen, but not more than six weeks before the first frost.

Mix bone meal into the soil at the bottom of the hole. If bulbs are going to be kept in a planting bed for more than one year, supply additional fertilizer. Mix five tablespoons of 10-10-10 soluble fertilizer (or equivalent bulb fertilizer) plus two cups of bonemeal per ten square foot area into the soil in the fall. Do not allow bulbs to touch fertilizer. The general rule of thumb for planting spring bulbs is to plant two to three times as deep as the bulbs is tall, measuring from the bottom of the bulb. This means most large bulbs like tulips or daffodils will be planted about 8 inches deep while smaller bulbs will be planted 3-4 inches deep. Summer bulbs have varied planting requirements, so consult the information supplied with the bulbs for proper planting depth.

Most bulbs are planted with the pointed end up and the root plate down. The best method of planting is to dig the entire bed to the proper depth, adding fertilizer, bone meal, and any amendments. Press the bulbs into the soil in the planting area and cover with soil.

Cover the bulb bed with two or three inches of mulch. Mulch will help minimize temperature fluctuation and maintain an optimal moisture level in the planting bed. Small, early-blooming bulbs should not be mulched.

Water the bulbs after planting. This helps settle the soil and provides moisture for the bulbs to start rooting. Fall-planted bulbs must root before cold weather. Avoid over-watering at planting time since this can result in bulb rot.

Maintaining

For both spring and summer bulbs, water whenever the soil is dry, any time after the flower buds first appear on the plant. Fertilize monthly from shoot emergence until the plants reach full flower.

After blooming, allow the leaves of the plant to wither and turn brown; do not cut or mow them. The plant needs the leaves to manufacture food (through photosynthesis) to be stored in the bulb for next year’s growth.

After the foliage is completely shriveled, spring bulbs can be left in place or dug up and replanted in the fall. Summer-flowering bulbs can be lifted (both bulblets and mother plant), stored over winter, and replanted in spring.

Dividing and Replanting Spring Bulbs

After about three years, spring bulbs should be “lifted” and divided. Signs that bulbs need to be lifted are overcrowding, multiple stems, and declining flowers.

The best time to divide and replant is after the foliage has yellowed. It is easier to locate bulbs when you can still see the leaves. Lift the bulb by placing a trowel beneath the plant and lifting. Cut off most of the brown foliage, but leave a couple of inches on the bulb.

When the plant has been lifted, remove the bulblets from the mother plant, then replant them separately, or at the same time you replant the mother plant, with the tips facing up. The bulblets will not bear flowers for about two years.

Tips for Gardening with Bulbs

  • During the growing season, water to keep soil moist but not soggy.
  • Weed frequently, as weeds take nourishment away from seedlings and bulbs.
  • Fertilize several times a season.
  • Daffodils are great for naturalizing (scattering clumps of plants throughout an area). They never have to be dug and are rabbit and deer resistant.
  • To disguise dying foliage, place bulbs behind other plants or interplant with annuals. Plant taller flowering bulbs behind low-growing plants or with groundcovers and perennials like hosta or daylilies.
  • For the most natural look, plant bulbs in clumps or swaths, not straight lines.
  • Some of the summer blooming bulbs like dahlias and gladioli occasionally need extra support to be able to remain erect.
Chester County Penn State Extension Office

601 Westtown Road, Suite 370
West Chester, PA 19380-0990
610-696-3500